1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus and an exposure method. More particularly, the present invention relates to a high-resolution catadioptric projection optical system suitable for a projection exposure apparatus used when semiconductor devices and liquid crystal display devices are fabricated in a photolithography step.
2. Related Background of the Invention
In a photolithography step for fabricating semiconductor devices or the like, a projection exposure apparatus for exposing a pattern image of a photomask or a reticle (hereinafter, generically referred to as “reticle”), through a projection optical system, on a wafer (or a glass plate) which is coated with photoresist and the like is used. Then, as a scale of integration of the semiconductor devices or the like is improved, resolving power (resolution) required for the projection optical system of the projection exposure apparatus is increased more and more. As a result of this, it is necessary to shorten wavelength of illumination light (exposure light) and to increase a numerical aperture (NA) of the projection optical system in order to satisfy a demand for the resolving power of the projection optical system.
For example, when exposure light having wavelength of 180 nm or less is used, it is possible to achieve a high resolution of 0.1 μm or less. However, if the wavelength of the illumination light is shortened, the light absorption will become remarkable, and types of glass materials (optical materials) that are durable enough to practical use will be limited. Particularly, when the wavelength of the illumination light becomes 180 nm or less, materials practically usable are limited to fluorite only. Consequently, in a dioptric (refractive) projection optical system, it becomes impossible to correct a chromatic aberration. Here, the dioptric optical system is an optical system including only transmissive optical members such as lens elements without including reflective mirrors (concave reflective mirrors or convex reflective mirrors) which have power.
As described above, the dioptric projection optical system composed of a single glass material has limitations on an allowable chromatic aberration, and it is inevitable to make a band of a laser light source extremely narrow. In this case, an increase in the cost of the laser light source as well as a decrease in the output thereof cannot be avoided. Moreover, in the dioptric optical system, it is necessary to arrange a large number of positive and negative lenses in order to approximate the Petzval Summation, which determines a field curvature, to 0. On the contrary, the concave reflective mirrors correspond to the positive lenses as optical elements converging light, but are different from the positive lenses in that no chromatic aberrations occur therein and the Petzval Summation takes negative values (incidentally, positive lenses take positive values).
In a so-called catadioptric optical system composed by combining the concave reflective mirrors and the lenses, in spite of a simple configuration thereof, a good correction of the chromatic aberrations and good corrections of various aberrations including the filed curvature are enabled by fully utilizing the above-described features of the concave reflective mirrors in an optical design. Accordingly, for example in International Publication WO01/65296, the applicants of the present application have proposed a three-time image-forming catadioptric optical system, which is composed of a refractive (dioptric) first image-forming optical system, a catadioptric second image-forming optical system and a refractive (dioptric) third image-forming optical system, as a high-resolution projection optical system suitable for the projection exposure apparatus.
However, in the conventional projection optical system disclosed in International Publication WO01/65296, a clear aperture diameter of a concave reflective mirror in the catadioptric second image-forming optical system configuring a protruding portion thereof is relatively large, and therefore mechanical stability in respect of vibrations is apt to be damaged. Moreover, a clear aperture diameter of a lens arranged adjacently to the concave reflective mirror is also relatively enlarged, and therefore it is not easy to procure and process a material that has a predetermined properties in the case where the lens is made of fluorite.
Moreover, because a distance (geometric distance) between an object surface and an image surface is relatively large in the conventional projection optical system, the mechanical stability in respect of vibrations is apt to be damaged. Then, because a distance between a reticle and a wafer is relatively enlarged when the projection optical system is mounted on the exposure apparatus, operativity is apt to be damaged, and height limitations of a clean room are apt to be imposed thereon. Furthermore, when the projection optical system is mounted on an exposure apparatus in which exposure light with wavelength of 200 nm or less is used, for example, a relatively long projection optical path will have to be filled with an inert gas, leading to a disadvantage in purging the inert gas.
Furthermore, in the conventional projection optical system, a distance from the concave reflective mirror in the catadioptric second image-forming optical system configuring the protruding portion thereof to a reference optical axis (optical axes of the first and third image-forming optical systems) is relatively large. As a result, the mechanical stability in respect of vibrations is apt to be damaged, and fabrication errors are apt to occur in assembling the second image-forming optical system. Moreover, when the projection optical system is mounted on the exposure apparatus in which the exposure light with the wavelength of 200 nm or less is used, for example, the relatively long projection optical path will have to be filled with an inert gas, leading to a disadvantage in purging the inert gas.